Planetcykler

Planetcykler is a term used in Danish-language astronomy to denote planetary cycles—recurring patterns in the motions and configurations of planets within the solar system. The term usually refers to periodicities that arise from orbital motion, gravitational perturbations, and resonance among planets. In practice, the scope can range from short synodic cycles to long-term secular variations.

Key types include synodic cycles, the time between conjunctions or oppositions of a pair of planets as observed from the Sun or Earth; sidereal (orbital) periods representing how long a planet takes to complete one orbit relative to fixed stars; and mean-motion resonances where planets exert regular gravitational pushes, producing stable relative configurations (for example, Pluto and Neptune in a 2:3 resonance, and the Jupiter–Saturn near a 5:2 resonance).

Secular cycles describe slow changes in orbital elements like eccentricity and inclination over millions of years due to mutual perturbations. Nodal precession and apsidal precession are examples of such long-term cycles that can influence climates on Earth on geological timescales.

Applications include constructing precise ephemerides for spacecraft navigation, planning observatories’ observing windows, and studying the dynamical stability of the solar system. Scientists use computational models and perturbation theory to identify cycle periods and their amplitudes, with data from missions and ground-based observations.

The term is not a formal designation in all astronomical catalogs, and in many sources it is described using broader terms such as planetary cycles or orbital dynamics.